1. Field of the Invention
This invention pertains to position determining devices, and in particular to devices that enable the position of an object or person to be determined relative to another person seeking said object, wherein a global positioning system receiver is used to determine the distance, direction and possible elevation difference between another global positioning system receiver.
2. Prior Art
Being able to determine the precise whereabouts of someone or something on or above the surface of the earth has long held promise for many purposes. Missing person searches would be much simpler if people who were lost had a transmitting device with them which constantly broadcast their precise position. Such a transmitter would be better than just a voice transmitter because the age of the people or their medical condition might prevent people from responding, or from responding in a helpful manner. However, numerous difficulties arise when actually searching for a transmitter which severely undermines the usefulness of such systems.
For example, U.S. Pat. No. 4,021,807 teaches how a transmitter hidden among stolen money could be used to locate those responsible for the theft and the money. A UHF homing device hidden among the money is capable of transmitting a signal which can be tracked by UHF tracking devices. Such a tracking device indicates whether the UHF homing signal is being transmitted from the front or rear, and from the left or right of a current position and orientation of the tracking device. Signal strength can also be used to give a crude estimation of distance between the tracking and homing devices if the signal is not too distorted by intervening structures.
The UHF homing signal and tracking devices comprise the same principle taught in U.S. Pat. No. 5,021,794. This patent teaches how a miniaturized transceiver carried by a child can be remotely activated by a parent to enable the child to be located by police cars with UHF trackers.
One of the drawbacks of such locator systems is that the position of the person or object is never known with any great degree of accuracy. A related issue is that the reliability of the signal received is also suspect, and can not be confirmed. Furthermore, a vehicle with a tracking device might circle a homing beacon many times before finding it due to the crude distance and direction indications of the technology.
Fortunately, a boon to precise location determining occurred when the United States saw fit to invest over $12 Billion in creating a network of 24 satellites in low earth orbit, each broadcasting precise timing signals from two on-board atomic clocks. Using precise and well-developed triangulation and quadrangulation formulas, a receiver that picks up signals from several satellites simultaneously can determine its position in global coordinates, namely latitude and longitude.
With this network orbiting overhead, a person anywhere on the earth has a 24 hour a day line-of-sight view to a sufficient number of satellites such that a person with a GPS receiver is able to determine their own longitude and latitude to within several meters, as well as their elevation. However, knowing your own position in longitude and latitude does not help others find you without extremely precise topographical or geophysical maps which also show longitude and latitude. Furthermore, the degree of precision in position determination is then only accurate to the resolution of the maps on hand. Nevertheless, the elements for a novel search and rescue system, as well as a general purpose locator, are made possible by the present invention utilizing GPS technology. Before the invention can be explained, however, a potential problem with GPS signals must first be explained.
In navigation, a method of guiding ships commonly used is dead-reckoning, whereby the known velocity and direction of travel of a ship from a known position such as a port is used to calculate the present position. The drawback is that the further a ship moves away from the known position, the less accurate the dead-reckoning position becomes. Inclement weather can further erode the accuracy of a ship's navigation, and endanger lives and property when traveling in close proximity to land. However, using a GPS receiver and a very accurate map with a sufficient degree of resolution, the movements of even a large vessel can be guided with a satisfactory degree of precision. The problem with GPS signals, surprisingly, arises from the high degree of precision that the system is able to provide.
It is the potential application of GPS technology to military uses which is responsible for the concern over GPS receiver accuracy. Specifically, precise positioning of targets can enable pinpoint accuracy in the delivery of highly destructive military payloads. Therefore, the possibility exists that our own satellite network could be used against the United States. For this reason, the GPS timing signals broadcast by the satellite network for commercial use are intentionally made less accurate than the encoded military signals. These timing and position errors are called Selective Availability (SA) and reduce the accuracy of civilian users to roughly 100 meters. While this inaccuracy is irrelevant on the high seas, coastal navigation or land-based applications such as search and rescue suffer, and potentially destroy the benefits of GPS technology.
To overcome the intentional errors introduced in the GPS timing signals, a system known as differential GPS (DGPS) was developed to reestablish accuracy for civilian users in a small, localized area such as coastal navigation. The system requires that a permanent GPS receiving and broadcasting station be established, and that the precise position of the station be determined. Using the fact that the errors introduced by a system of satellites will be the same errors transmitted to all receivers in a localized area, a mobile GPS receiver in range of the permanent station can determine its position and achieve the same degree of accuracy enjoyed by the military. This is accomplished by having the permanent station calculate the error introduced by the GPS satellites by comparing the signal received with the actual known position. This error factor can be transmitted to and used by all mobile receivers within the vicinity of the permanent station to determine their position accurately to within several meters instead of 100 meters. Of course, the accuracy of this DGPS determined position decreases the further away that a GPS receiver is from the permanent GPS receiving and broadcasting station.
Another form of differential GPS position determination has also substantially increased the usefulness of GPS receivers. As taught in Smith, U.S. Pat. No. 5,408,238, a comparison of absolute GPS determined locations can be used to determine the relative position or location of the GPS devices relative to each other. This comparison eliminates the need for a permanent base station which transmits an error correction factor because the absolute position of the GPS receivers is relevant only so far in that they are compared to each other to provide a relative position difference.
Returning now to our problem of locating a missing person, the exact longitude and latitude provided by DGPS is not often useful without very precise maps of sufficient resolution and of the area in question. Elevation may also play a very important factor if someone is lost in mountainous terrain. Therefore, it would be an advance over the prior art if a graphical interface could be provided for a differential or relative position GPS position detection system which would intuitively provide searchers a distance measurement and direction. It would also be an advantage if the graphical interface provided position information accurate to several meters using only GPS signals and positions determined by the systems GPS receivers, regardless of whether a permanent station is nearby providing GPS SA error compensation information. It would also be an advance over the prior art if the difference in elevation between the searchers and the lost person could be provided to that same degree of accuracy.